Alot depends on your age and finance's.
Carnival is normally for the young and is a cheaper lines.
RCL is for the young but is pricier.
NCL falls right in the middle from what I have heard and read.
Princess also falls into the middle but is a little more refined.
HAL, from what I have read is for the older crowd.
Celebrity is pricey and has a somewhat older crowd.
I don't think there is such a thing as a bad cruise. Like some of the others have said, location, ports to visit, time of year, ect...all play into choosing a cruise.
I highly suggest the Carnival Valor for ones first cruise. Eastern Carribean to St. Thomas and St. Martin is a great place to start. Prices are normally very reasonable too.